# Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions

At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the Quark-Gluon Plasma (QGP) [1]. Such an extreme state of strongly-interacting QCD (Quantum Chromo-Dynamics) matter is produced in the laboratory with high-energy collisions of heavy nuclei, where an enhanced production of strange hadrons is observed [2-6]. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions [7], is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions [8,9]. Yet, enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity pp collisions. We find that the integrated yields of strange and multi-strange particles relative to pions increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with p-Pb collision results [10,11] indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.

Figures

## Figure 1

 $\pt$-differential yields of ${\rm K}^{0}_{S}$, $\Lambda+\overline{\Lambda}$, $\Xi^{-}+\overline{\Xi}^{+}$ and $\Omega^{-}+\overline{\Omega}^{+}$ measured in $\left|y\right| < 0.5$. The results are shown for a selection of event classes, indicated by roman numbers in brackets, with decreasing multiplicity. The error bars show the statistical uncertainty, whereas the empty boxes show the total systematic uncertainty. The data are scaled by different factors to improve the visibility. The dashed curves represent Tsallis-Lévy fits to each individual distribution to extract integrated yields. The indicated uncertainties all represent standard deviations.

## Figure 2

 $\pt$-integrated yield ratios to pions (${\pi}^{+}+{\pi}^{-}$) as a function of $\langle {{\rm d}N_{\rm ch}/{\rm d}\eta} \rangle$ measured in $\left|y\right| < 0.5$. The error bars show the statistical uncertainty, whereas the empty and dark-shaded boxes show the total systematic uncertainty and the contribution uncorrelated across multiplicity bins, respectively. The values are compared to calculations from MC models [30-32] and to results obtained in p-Pb and Pb-Pb collisions at the LHC [6, 9, 11]. For Pb-Pb results the ratio $2 \Lambda$/(${\pi}^{+}+{\pi}^{-}$) is shown. The indicated uncertainties all represent standard deviations.

## Figure 3

 Particle yield ratios $\Lambda/{\rm K}^{0}_{S} = (\Lambda+\overline{\Lambda})/2{\rm K}^{0}_{S}$ and ${\rm p}/{\pi} = ({\rm p}+\overline{\rm p})/({\pi}^{+}+{\pi}^{-}$) as a function of $\langle {\rm d}N_{\rm ch}/{\rm d}\eta \rangle$. The yield ratios are measured in the rapidity interval $\left|y\right| < 0.5$. The error bars show the statistical uncertainty, whereas the empty and dark-shaded boxes show the total systematic uncertainty and the contribution uncorrelated across multiplicity bins, respectively. The values are compared to calculations from MC models [30-32] in pp collisions at $\sqrts = 7$ TeV and to results obtained in p-Pb collisions at the LHC [10]. The indicated uncertainties all represent standard deviations.

## Figure 4

 Particle yield ratios to pions normalised to the values measured in the inclusive INEL$>$0 pp sample. The results are shown for pp and p-Pb collisions, both normalised to the inclusive INEL$>$0 pp sample. The error bars show the statistical uncertainty. The common systematic uncertainties cancel in the double-ratio. The empty boxes represent the remaining uncorrelated uncertainties. The lines represent a simultaneous fit of the results with the empirical scaling formula in Equation (1). The indicated uncertainties all represent standard deviations.